Objective: To gain insight into the effects of duration of type 2 diabetes on insulin secretion in patients with type 2 diabetes mellitus.
Methods: C-peptide concentrations were measured every 2 years before and after intravenous injection of 1 mg of glucagon in 89 patients with type 2 diabetes (51 men and 38 women) as part of the Rochester Diabetic Neuropathy Study in those subjects who participated in follow-up (median, 12 years; range, 6 to 14).
Results: Although insulin secretion decreased over time (P<0.001) in the group as a whole, both the pattern and the rate of decline in C-peptide concentration differed considerably among the study subjects. Insulin secretion, whether measured as fasting C-peptide, 6-minute C-peptide, or postglucagon increment in C-peptide concentrations, declined with increasing duration of diabetes in approximately half of the patients but either increased or remained essentially constant over time in the other half. The decrease in insulin secretion was not associated with a deterioration in glycemic control because hemoglobin A1c also declined (P<0.005) during the same interval.
Conclusion: We conclude that insulin secretion decreases over time in many patients with type 2 diabetes. Because the rate of decline is variable, the predictive value of any single measurement is limited. These data indicate that although a decrease in insulin secretion over time is characteristic of type 2 diabetes mellitus, it is not inevitable.